For example, in a vehicular seat described in WO 2010/131322, a side airbag module is attached to an outer side of a side frame portion of a seat back frame in a seat width direction. Besides, a mechanical side support adjustment mechanism is attached to an inner side of the side frame portion in the seat width direction. This side support adjustment mechanism drives a paddle constituting a skeleton of a side support with the aid of a drive mechanism, thereby displacing the side support in the seat width direction. Part of the aforementioned paddle is located in an inflation and deployment region of a side airbag. At the time of inflation and deployment of the side airbag, the paddle receives an inflation pressure of the side airbag, and is displaced inward in the seat width direction. Thus, the side airbag is deployed in a favorable manner.
On the other hand, in a vehicular seat described in Japanese Patent Application Publication No. 2008-143363 (JP 2008-143363 A), a side airbag module is attached to an outer side of a lateral portion (a side frame portion) of a frame unit of a seat back in a seat width direction. Besides, an air bag-type side support adjustment mechanism is attached to an inner side of the side frame portion in the seat width direction. In this side support adjustment mechanism, an air bag (a bag body) is fixed to the side frame portion via a metallic support bracket. When air is introduced into this air bag, the air bag inflates while receiving a reaction force from the support bracket, and displaces a lateral portion support block (a side support) of the seat back inward in the seat width direction.